Copper is a material finding increasing use as a conductive metal in interconnections for high performance integrated circuit applications. It has lower electrical resistance and better electromigration performance in comparison with many other metals, including aluminum. Copper can be readily deposited by chemical vapor deposition, physical vapor deposition, electroless deposition, and electroplating. Chemical vapor deposition and electroplating techniques provide good step coverage, with electroplating techniques usually being the lower cost of the two.
Copper interconnects are typically formed within or adjacent various interlevel dielectric layers which commonly comprise silicon dioxide. Unfortunately, copper can migrate or diffuse into silicon dioxide when subjected to high temperature or high bias. Copper diffusion into silicon dioxide tends to one or both of deteriorate the insulative characteristics of the silicon dioxide and may cause device leakage currents. Further, copper does not readily adhere to silicon dioxide surfaces. To overcome these issues, adhesion promoter/diffusion barrier layers have been provided intermediate copper interconnects and interlayer dielectrics such as silicon dioxide. Example adhesion promoter/diffusion barrier layer materials include physical vapor deposited tantalum or tantalum nitride. A copper-comprising seed layer is then typically deposited over the barrier layer to provide a highly conductive surface for subsequent copper deposition using electroplating.
Recently, the copper seed layer has been formed to comprise an alloy of copper and aluminum or an alloy of copper and magnesium. It has been discovered that subsequent heating of the substrate has a tendency to cause the aluminum and/or magnesium atoms within the seed layer to migrate into the electroplated copper and to the elevationally outermost surface thereof. Such desirably forms a conductive passivation layer which can serve to both protect the outer copper surface and provide a good conductive interconnect for metal which might be deposited subsequently in conductive connection therewith. Yet, the migration of the aluminum and/or magnesium atoms to the outer surface of the electroplated copper is not very controlled, and does not produce readily repeatable results. Embodiments of the invention were motivated in addressing the above-identified issues. However, the artisan will appreciate that embodiments of the invention will have applicability beyond addressing the above-identified issues, and as is explained and expanded upon below.